A large number of organic EL devices each using an organic substance have been developed because of their potential to find applications in solid light emission type, inexpensive, large-area, full-color display devices. In general, an EL device is constituted of a light emitting layer and a pair of opposing electrodes between which the layer is interposed. Light emission is the following phenomenon: when an electric field is applied between both the electrodes, an electron is injected from a cathode side and a hole is injected from an anode side, and, furthermore, the electron recombines with the hole in the light emitting layer to produce an excited state, and energy generated upon return of the excited state to a ground state is emitted as light.
A conventional organic EL device has been driven at a voltage higher than the voltage at which an inorganic light emitting diode is driven, and has had emission luminance and luminous efficiency lower than those of the diode. In addition, the properties of the device have deteriorated remarkably, so the device has not been put into practical use. Although a recent organic EL device has been gradually improved, additionally high luminous efficiency and an additionally long lifetime of the device are requested. For example, a technique involving the use of a single monoanthracene compound as an organic light emitting material has been disclosed (Patent Document 1). However, the technique provides a luminance as low as 1,650 cd/m2 at a current density of, for example, 165 mA/cm2, and provides extremely low efficiency, specifically, 1 cd/A, so the technique is not practical. In addition, a technique involving the use of a single bisanthracene compound as an organic light emitting material has been disclosed (Patent Document 2). However, even the technique provides an efficiency as low as about 1 to 3 cd/A, so an improvement for putting the technique into practical use has been demanded. Meanwhile, a long-lifetime organic EL device obtained by adding, for example, styrylamine to a distyryl compound to be used as an organic light emitting material has been proposed (Patent Document 3). However, the device does not have a sufficient lifetime, and the additional improvement of the device has been demanded.
In addition, techniques each involving the use of each of a monoanthracene or bisanthracene compound and a distyryl compound in an organic light emitting medium layer have been disclosed (Patent Document 4). However, in each of those techniques, the conjugate structure of the styryl compound lengthens the wavelength of an emission spectrum, with the result that a color purity is deteriorated. Further, Patent Document 5 discloses a blue light emitting device using a diaminochrysene derivative.
The blue light emitting device is excellent in luminous efficiency. However, the device does not have a sufficient lifetime, and the additional improvement of the device has been demanded.
In addition, furthermore, Patent Document 6 discloses an organic EL device using a tetraarylethene derivative. However, when the tetraarylethene derivative is used in the light emitting layer of the device, a luminescent color becomes a cyan color, so blue light having a high color purity cannot be emitted.
Patent Document 1: JP 11-3782 A
Patent Document 2: JP 8-12600 A
Patent Document 3: WO94/006157 A
Patent Document 4: JP 2001-284050 A
Patent Document 5: WO04/044088 A
Patent Document 6: JP 11-260551 A